This invention relates to a membrane plate for a plate filter press composed of a plurality of such plates. Each plate has a plate wall with a continuous plate frame which, together with adjacent filter plates on both sides of the plate, defines a chamber. A press membrane that can be acted upon by a pressure medium is disposed on at least one side of the plate. The press membrane has a continuous peripheral bead which is fixed in a groove in a transitional zone between the plate frame and the plate wall.
A membrane plate of the above type is described in German Utility Model Patent DE-GM No. 84 02 617.0. In this known membrane plate, the peripheral bead has a dovetail-like cross section, anchored in a form-fitting manner in a groove that also has a dovetail-like cross section. The portion of the peripheral bead that is joined to the edge of the membrane protrudes slightly beyond the plane defined by the contact face of the plate frame, when the chamber is open. The filter cloth is located on either side, facing the chamber, covering both the contact face of the plate frame and the entire press membrane. When the chamber is closed, the peripheral bead is pressed into the dovetail-like groove and deformed such that the contact faces of the frame, or the filter cloths resting on the frame, are pressed against one another by the closing force of the press.
This kind of fastening has certain disadvantages. Firstly, mounting a press membrane, or removing a worn press membrane, can be done only by exerting considerable force because of the form-fitting fixation of the peripheral bead in the groove, and there is always the danger that if tools are used, the peripheral bead will be nicked and, given the considerable deformation that occurs, this becomes the beginning of an ever-enlarging fissure.
A further disadvantage is that when the filter plates are pressed against one another, the portion of the peripheral bead that is anchored in the groove cannot deflect into the groove to a sufficient extent. Therefore, in the course of the resulting laterally deflecting deformation toward the press membrane, localized shear strains can arise even in the elastic material of which the press membrane is made, which is typically rubber. These strains can exceed the permissible limits of the rubber material.
Still another disadvantage of this known press membrane, which has a cup-shaped form, is that for the pressing operation the press membrane is inverted into the chamber, and so, because of the resultant changes in its original geometry in the transitional zone between the clamped portion and the exposed surface of the membrane, the rubber material is subjected not only to the shear strains already mentioned, but also to tensile and compressive strains. The service life of the press membrane is shortened as a result.